1. Field of the Invention
The present invention relates to an objective lens disposed at a front end portion of an endoscope; and, in particular, to an objective lens for an endoscope, which matches a high-resolution imaging device used for transmitting image information.
2. Description of the Prior Art
In general, a lens having a wide angle of view and a long back focus is required as an objective lens for an endoscope. Also, for carrying out highly accurate viewing and precise diagnosis, it is important to correct chromatic aberration. For example, an objective lens for an endoscope has already been disclosed in commonly-assigned Japanese Unexamined Patent Publication No. 2-188709 as one satisfying the requirements mentioned above.
Recently, however, as CCD devices and image fibers used for image transmission have been attaining higher resolutions, lenses with higher resolutions have been demanded. In view of a higher resolution in the device used for image transmission, the endoscope objective lens disclosed in Japanese Unexamined Patent Publication No. 2-188709 needs to further improve its chromatic aberration in magnification.
On the other hand, there has been known an objective lens using an optical system equipped with a diffraction optical surface (hereinafter referred to as "diffraction optical element") as means for correcting chromatic aberration. The diffraction optical element has an Abbe number of .nu..sub.d =-3.45, whereby its dispersion is greater than that of a normal glass lens by one digit while having an opposite polarity. Hence, it is advantageous in that chromatic aberration can effectively be corrected to an extent which is unpredictable from the normal lens. An example of such an endoscope objective lens is disclosed in Japanese Unexamined Patent Publication No. 10-197806.
The endoscope objective lens disclosed in Japanese Unexamined Patent Publication No. 10-197806, however, may be disadvantageous in that diffracted light of different orders forming unnecessary light in terms of design generated due to the use of a diffraction optical element, flare light occurring due to errors in manufacture or the like, and so forth affect the achievement of a higher resolution.